The present invention relates to a charging device for charging such a battery, by an output generated from a constant current source, as is suitable for use in an AC power adapter used as a power supplying device in an electronic apparatus such as a portable video tape recorder.
Hitherto, in order to charge a plurality of batteries (for example, an NiCd battery or cell), this plurality of batteries (including a battery pack which contains many NiCd batteries) have been charged successively one after another, and separately from each other in one way. In this case, a so-called full-charge detection circuit is required because, when one battery which is being charged is found to be already fully charged, then the charging of this battery should be stopped so that a next battery is started to be charged. Japanese Patent Publication Tokkosho No. 60-18177 discloses one example of the full-charge detection circuit referred to above. The prior art device disclosed in Tokkosho No. 60-18177 is, with noting the fact that the charging voltage characteristic is gradually dropped after passing its peak point, so constructed as to decide that the battery is fully charged through detection of the voltage drop by a predetermined amount (.DELTA.v) compared with the voltage at the peak point.
Moreover, in another prior art device disclosed in Japanese Patent Laid-Open Publication (unexamined) Tokkaisho No. 56-110446, a charging device is so designed that a plurality of NiCd batteries are separated into blocks. Every time one battery block is detected to be fully charged, another battery block is started for the charging. Or when a short-circuited battery is connected, this is detected, and a next battery block is started to be charged. The detected of the short-circuited battery is conducted in the manner that while the charging voltage of the battery is compared at all times with a predetermined reference voltage, and when it is found to be lower than the reference voltage, the battery is determined to be short-circuited. However, the prior art has such a drawback that when the battery is an over-discharged battery, the battery might be erroneously determined to be short-circuited.
In the case where a fully-charged battery which is already in a fully-charged condition is re-charged, with employing the prior art fully-charge detection circuit described above, the charging characteristic shows its peak one minute after the start of the charging, and it falls down linearly thereafter. The voltage drop by .DELTA.v is detected in the middle of the linear falling of the charging characteristic, leading to the stop of the charging. On the other hand, the charging characteristic of the over-discharged battery is such that after passing its peak, it once drops to reach a steady state.
Therefore, in the process to reach the steady state, the voltage drop of .DELTA.v is detected, which might be erroneously determined as that the battery is fully charged, resulting in an undesired stoppage of the charging. With consideration into the above-described fact, it is necessary to extend the time period during which the .DELTA.v voltage drop is prohibited from being detected before the charging characteristic reaches the steady state (e.g., for 5 minutes after the start of the charging). However, if the prohibition time is extended, the full-charge detection of a fully-charged battery when it is re-charged is delayed, and consequently the battery is over-charged from an increased period of time, resulting in an inferior charging.
In the meantime, when a plurality of NiCd batteries are connected in series to an individual battery, that is, a battery pack is employed, and if some of the NiCd batteries are short-circuited, namely, the battery pack is partially short-circuited, such as phenomenon that the charging voltage exceeds the reference voltage which is set to detect when the battery is completely short-circuited, and accordingly it becomes impossible to detect that the battery pack is partially short-circuited.